If baby is on full formula, then you must give her water after every feed, or else she will have constipation. Full fm babies should poo once a day. If you gently press your babies tummy and its hard, then she is probably having constipation. If tummy not hard then its not constipation.
My first son also had constipation and my mother in law actually inserted a thin piece of butter up his anus on/after the third day. Its like a natural enema...a bit uncomfortable for him but not painful. 2 hours later, he pooed. My colleague's baby also had constipation and they didn't know what to do so they brought him to doc after 5 days. The doctor gave enema and it was very painful...the baby ended up with a phobia of pooing and only poo normally when he was 1+ yrs old.